RFR: 7903667: Functional interface name nested inside function pointer class should be mangled

[Maurizio Cimadamore]

This simple PR adds name mangling for the functional interface nested inside the function pointer class, in case the latter is called "Function".

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Commit messages:
 - Initial push

Changes: https://git.openjdk.org/jextract/pull/215/files
 Webrev: https://webrevs.openjdk.org/?repo=jextract&pr=215&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903667
  Stats: 73 lines in 3 files changed: 64 ins; 1 del; 8 mod
  Patch: https://git.openjdk.org/jextract/pull/215.diff
  Fetch: git fetch https://git.openjdk.org/jextract.git pull/215/head:pull/215

PR: https://git.openjdk.org/jextract/pull/215